This invention relates to implantable diagnostic and tissue stimulation devices such as implantable pacemakers, cardioverters and defibrillators, implantable monitoring devices and implantable drug dispensers, and more particularly to rate-responsive implantable pacemakers that vary their pacing rate as a function of the patient's metabolic demand for oxygenated blood.
Early pacemakers provided a fixed rate stimulation pulse generator that could be reset on demand by sensed atrial and/or ventricular depolarizations. Modern pacemakers include complex stimulation pulse generators, sense amplifiers and leads which can be configured or programmed to operate in single or dual chamber modes of operations, delivering pacing stimuli to the atrium and/or ventricle at fixed rates or rates that vary between an upper rate limit and a lower rate limit.
More recently, single and dual chamber pacemakers have been developed that respond to physiologic sensors which, with greater or lesser degree of specificity, sense the body's need to deliver more or less oxygenated blood to the cardiovascular system. For example, rate responsive pacing systems have been developed and marketed which rely upon the central venous blood temperature, as measured in the right ventricle. Such pacemakers are disclosed in U.S. Pat. No. 4,688,573 issued to Alt and U.S. Pat. No. 4,543,954 issued to Cook et al.
It has been proposed in the prior art to measure the time interval between an atrial stimulation pulse, A, and the responsive atrial or ventricular depolarization, P or R, respectively, as an indication of the physiological demands placed on the heart. The time interval between a ventricular stimulation pulse, in, and the responsive ventricular depolarization, R, is also proposed to be measured and used as an indication of physiologic need. The atrial depolarization is sensed by detecting a P-wave wherein the ventricular depolarization is sensed by detecting an R-wave. U.S. Pat. No. 4,712,555 sets forth such a system which includes the further method of measuring the A-P or A-R to ascertain whether the intervals are increasing or decreasing. If over several heart cycles or beats, an increase or decrease in these measurements is detected, the pacing interval set by the pacemaker is adjusted in an appropriate direction in order to adjust the heart stimulation rate accordingly. In the '555 patent, atrial and ventricular bipolar sense/pace electrodes are distributed in the right atrium and ventricle of the heart, and the measurements are taken by way of conventional pacemaker sense amplifiers.
It has also been proposed in the prior art to measure the evoked potential following the application of a pacing stimulus and to employ its amplitude and/or configuration to regulate automatically the implantable cardiac pacemaker's output characteristics, such as rate. U.S. Pat. No. 4,759,367 describes such a cardiac pacing system which combines unipolar and bipolar electrode configurations to detect the evoked potential and apply the pacing stimulus respectively. It is suggested therein that a bipolar lead may be placed in the atrium or the ventricle or a pair of such bipolar leads may be placed in either chamber for dual chamber pacing.
In the '367 patent, it is proposed that the magnitude of the evoked response be integrated over time to obtain a repolarization gradient value. The rate control algorithm involves comparing the measured repolarization gradient magnitude of the current cardiac cycle with a stored repolarization gradient magnitude of at least one previous cardiac cycle. It is known that with conventional sense amplifiers of the type shown in the '367 patent having a high input impedance that one may detect and integrate the varying amplitude of the relatively wide QRST waveform relatively easily. However, it is difficult to use such sense amplifiers to consistently identify the same point in the QRST wavefront in order to develop a time interval representative of the myocardial conduction velocity.
In this regard, attention is also drawn to Rickards U.S. Pat. No. 4,228,803 which relies upon the detection of time intervals following the delivery of a pacing stimulus to develop a rate control signal to either increase or decrease the pacing rate. In the '803 patent, the system measures the stimulus to repolarization (i.e. T-wave) interval, otherwise known as the QT interval. Again, in these patents and in the products which have been developed and marketed implementing them, conventional high input impedance sense amplifiers have been employed to detect the evoked response.
In U.S. Pat. No. 4,750,494 issued to King, conventional sense amplifiers are used to detect arrival times of depolarization wavefronts at multiple electrode sets are in order to measure changes in depolarization wavefront propagation direction, as part of a system for detecting impending fibrillation. If the net direction of the depolarization wavefront changes by more than a predetermined amount, the device determines that fibrillation is likely to be imminent. A reduction or increase in myocardial conduction velocity without a change in propagation direction will apparently not be detected.
In the specific environment of the present invention where it is necessary to accurately and reproductively measure very small time intervals, prior art sense amplifiers are not optimal. The presentation of the atrial or ventricular depolarization wave front across the two spaced electrodes varies from patient to patient and, from time to time, in the same patient. This presentation may affect the rise time amplitude and polarity of the electrical depolarization wave front signal and affect the trigger point of the sense amplifier, leading to inaccurate interval data.